The present invention concerns a method and apparatus for manufacturing a laminated tissue product which has a consistent ply bond strength for, among other things, improved flexographic printing ability.
In the field of making roll stock of laminated tissue for use in making paper products such as printed napkins, it is important to be able to achieve a desired or predetermined ply bond strength. It is also important to be able to attain a consistent ply bonding strength throughout the ply layers. As discussed in U.S. Pat. No. 6,136,422, which is incorporated herein by reference, a consistent strength in the ply bonding assists in providing an efficient method to produce printed napkins by flexographic printing. For example, the ability to maintain a consistent predetermined bond strength minimizes breaks in the web during printing and converting. This, in turn, reduces waste and increases in operating efficiency.
While U.S. Pat. No. 6,136,422 purports to provide an improved spray bonding technique, the very nature of spray bonding presents several disadvantages for manufacturing tissue laminates for use in flexographic printing processes and other manufacturing processes. First, spraying involves the random application of adhesive droplets to the ply to create a ply bond. The random nature of this application results in an uneven application of adhesive across the web with small voids of adhesive appearing throughout the web. This creates an inconsistent bond strength in the web which may result in breaks during printing or in other processes which require the web to travel through machinery at high speeds. In addition, adhesive will often accumulate on or around the spray nozzles and drip onto the web in undesirable amounts. This results in the adhesive penetrating multiple layers of the web which results in a situation commonly referred to as blocking. Blocking is one of the main causes of web breaks in paper napkin manufacturing.
Equally important, tissue laminates made from spray bonding further suffer from post-printing deficiencies. For example, when the tissue is printed, the absorbent tissue plies are re-wetted and swelling occurs between the bonding points. Because the adhesive points in a spray bonding process are random in nature, random amounts of swelling will appear along the web with large amounts appearing in some areas and smaller amounts appearing elsewhere. When the tissue re-drys, the printed surface will often appear uneven or distorted as a result of the uneven swelling.
Yet another disadvantage of using spray bonding to form the adhesive bond is that puckering or wrinkling will often occur on the non-printed side of the tissue in the machine direction after printing. This negative effect is, again, the result of the random application of adhesive droplets to form the ply bond. The random application of droplets by spraying results in small pathways being formed in the ply bond which align in the machine direction. A result of these uninterrupted pathways is wrinkling in the post printed product due to a lack of uniform bonding area which prevents a wrinkle from occurring.
The present invention overcomes the above-mentioned deficiencies in spray bonding. In particular, the present invention overcomes the deficiencies in spray bonding by printing onto the tissue plies a uniform layer of adhesive. In one embodiment, the uniform layer is applied over the entirety of the web. This creates a laminate with no voids in the adhesive layer.
In another embodiment, the adhesive is applied in a pattern which is non-linear and which forms random cells. This embodiment of the present invention provides the same benefits discussed above along with increased loft, if desired.